Such a service usually involves the use of an IT communication protocol called real-time transport protocol (RTP). With this protocol, the data are transmitted in the form of packets and a mechanism for retransmitting lost data packets is set up to ensure a sufficient level of quality at reception.
At present, an additional passband is used solely in cases of loss, in order to retransmit the lost packets. This dedicated passband is generally small in relation to the passband allocated to the VoD service. The application JP 2003 046593 illustrates an example of use of such a method for retransmitting lost packets, said example being shown in FIG. 1.
In this FIG. 1, packets P(i) are transmitted, at a given speed D1, in a data stream in real time. When some packets PP(i) in this stream are lost, they are later retransmitted with other packets in real time, using an overhead at discrete times in order to attain an increased overall transmission speed D2.
Thus, during the time interval [t1;t2] during which the lost packets PP(i) are retransmitted, these retransmitted packets PP(i) are transmitted at the same time as other packets in real time P′(i), without the data stream in real time being modified by the presence of these retransmission packets. At reception, a buffer memory allows the retransmitted data to be stored. The size of the buffer memory and the value of the additional passband determine the maximum size of the losses that it is possible to correct. In particular, the smaller the value of the additional passband (i.e. the difference between D2 and D1 in FIG. 1), the more time will be necessary in order to retransmit the lost packets.
By way of example, with an additional passband of 20% and losses of 80 ms, a buffer memory of 640 ms is required in order to correct the losses, if the time between the retransmission request and the reception of the first retransmitted packet is 240 ms, this being able to be reproduced every 720 ms.
Thus, it can be seen that the losses that it is possible to correct are relatively short and that they must not be too frequent. This correction mechanism is therefore perfectible.